Method of transmitting reference signals in a wireless communication having multiple antennas

ABSTRACT

A method of transmitting signals by a transmitting end in a wireless communication system comprises sharing control information related to reference signal with a receiving end; generating one or more precoded reference signals considering a given rank; allocating the one or more precoded reference signals to have a specific pattern within a subframe, wherein the specific pattern is varied depending on the control information; and transmitting the subframe through multiple antennas to the receiving end.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/567,665, filed on Sep. 25, 2009, now U.S. Pat. No. 8,428,018, whichclaims the benefit of U.S. Provisional Application No. 61/100,271, filedon Sep. 26, 2008, and also claims the benefit of earlier filing date andright of priority to Korean Patent Application No. 10-2008-0132995,filed on Dec. 24, 2008, the contents of which are all herebyincorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of performing communication ina wireless communication system, and more particularly, to a method oftransmitting reference signals in a wireless communication system havingmultiple antennas.

2. Discussion of the Related Art

A 3rd Generation Partnership Project (3GPP) wireless communicationsystem based on the Wideband Code Division Multiple Access (WCDMA) radioaccess technology has been widely developed. A High Speed DownlinkPacket Access (HSDPA) that can be defined as a first evolution stage ofthe WCDMA provides the 3GPP with a radio access technology having highcompetitiveness in the mid-term future.

An example of the radio access technology for providing highcompetitiveness in the long-term future includes an Evolved UniversalMobile Telecommunications System (E-UMTS). The E-UMTS is an evolvedversion of the conventional WCDMA UMTS, and its basic standardization isin progress under the 3GPP. The E-UMTS is also referred to as a LongTerm Evolution (LTE) system. For details of the technical specificationsof the UMTS and E-UMTS, refer to Release 7 and Release 8 of “3rdGeneration Partnership Project; Technical Specification Group RadioAccess Network.”

The E-UMTS includes a User Equipment (UE), a base station, and an AccessGateway (AG) which is located at an end of a network (E-UTRAN) andconnected to an external network. Generally, the base station cansimultaneously transmit multiple data streams for a broadcast service, amulticast service and/or a unicast service. In the LTE system, anorthogonal frequency divisional multiplexing (OFDM) scheme and amultiple-input multiple-output (MIMO) scheme are used todownlink-transmit various services.

The OFDM scheme represents a high-speed data downlink access system. TheOFDM scheme has an advantageous of high spectral efficiency in that thewhole spectrum as allocated can be used by all base stations. For OFDMmodulation, a transmission band is divided into a plurality oforthogonal subcarriers in a frequency domain and a plurality of symbolsin a time domain. Since the OFDM scheme divides the transmission bandinto a plurality of subcarriers, a bandwidth per subcarrier is reducedand a modulation time per carrier is increased. Since the plurality ofsubcarriers are transmitted in parallel, a transmission rate of digitaldata or symbols of a specific subcarrier is more lowered than that of asingle carrier.

The MIMO scheme is used for a communication system with a plurality oftransmitting and receiving antennas. The MIMO scheme can linearlyincrease channel capacity without additional increase of a frequencybandwidth as the number of transmitting and receiving antennasincreases. Examples of the MIMO scheme include a spatial diversityscheme and a spatial multiplexing scheme, wherein the spatial diversityscheme can enhance transmission reliability using symbols which havepassed through various channel paths, and the spatial multiplexingscheme is to increase a transmission rate by simultaneously transmittingrespective data streams from the respective antennas using a pluralityof transmitting antennas.

Also, the MIMO scheme can be divided into an open-loop MIMO scheme and aclosed-loop MIMO scheme depending on whether a transmitter knows channelinformation. In the open-loop MIMO scheme, the transmitter does not knowchannel information. Examples of the open-loop MIMO scheme include PARC(per antenna rate control), PCBRC (per common basis rate control), BLAST(Bell Laboratories Layered Space Time), STTC (space time trellis code),random beamforming, etc. On the other hand, in the closed-loop MIMOscheme, the transmitter knows channel information. Throughput of theclosed-loop MIMO scheme depends on how exactly the transmitter knows thechannel information. Examples of the closed-loop MIMO scheme includePSRC (per stream rate control), TxAA (Transmit Antenna Array), etc.

The channel information means radio channel information (for example,attenuation, phase shift, or time delay, etc.) between a plurality oftransmitting antennas and a plurality of receiving antennas. In the MIMOscheme, various stream paths exist in accordance with combination of aplurality of transmitting and receiving antennas. The MIMO scheme hasfading characteristics that a channel status is irregularly changed intime/frequency domains depending on time due to multi-path time delay.Accordingly, a receiver obtains channel information through channelestimation. The channel estimation is to estimate channel information torecover a distorted transmission signal. For example, the channelestimation means that amplitude and reference phase of carriers areestimated. Namely, the channel estimation is to estimate frequencyresponse of a radio interface or a radio channel.

An example of a channel estimation method includes a method ofestimating a reference value based on reference signals (RS) of severalbase stations using a two-dimensional channel estimator. The referencesignals (RS) mean symbols having no real data but having high output toassist carrier phase synchronization and acquisition of base stationinformation. The transmitter and the receiver can perform channelestimation using such reference signals (RS). Channel estimation throughthe reference signals (RS) means that a channel is estimated through asymbol commonly known by the transmitter and the receiver and data arerecovered using the estimated result. The reference signals (RS) arealso referred to as pilots.

The MIMO scheme supports a time division duplexing (TDD) mode and afrequency division duplexing (FDD) mode. Since forward link transmissionand reverse link transmission lie on the same frequency domain in theTDD mode, a forward link channel can be estimated from a reverse linkchannel by the reciprocity principle.

Although the wireless communication technology developed based on WCDMAhas been evolved into LTE, request and expectation of users andproviders have continued to increase. Also, since another wirelessaccess technology is being continuously developed, new evolution of thewireless communication technology is required for competitiveness in thefuture. In this respect, reduction of cost per bit, increase ofavailable service, use of adaptable frequency band, simple structure,open type interface, proper power consumption of user equipment, etc.are required.

In this respect, standardization of advanced technology of LTE is inprogress under the 3rd Generation Partnership Project (3GPP). Herein,this technology will be referred to as “LTE-Advance” or “LTE-A.” One ofimportant differences between the LTE system and the LTE-A system is thenumber of antennas for transmission. Currently, the LTE system aims tosupport a single antenna. On the other hand, the LTE-A system aims tosupport multiple antennas that reach maximum four antennas. Accordingly,the LTE-A system should support transmission of reference signals formaximum four antennas. Particularly, in the LTE-A system, a method forsupporting multi-user MIMO (MU-MIMO) is discussed.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method oftransmitting reference signals in a wireless communication system havingmultiple antennas, which substantially obviates one or more problems dueto limitations and disadvantages of the related art.

An object of the present invention is to provide a method of efficientlytransmitting reference signals in a wireless communication system havingmultiple antennas.

Another object of the present invention is to provide a method offlexibly controlling a pattern of reference signals depending on acommunication condition.

Still another object of the present invention is to provide a signalingmethod for flexibly controlling a pattern of reference signals.

Further still another object of the present invention is to provide amethod of flexibly controlling a pattern of reference signals toefficiently support SU-MIMO and/or MU-MIMO.

Further still another object of the present invention is to provide asignaling method for flexibly controlling a pattern of reference signalsto efficiently support SU-MIMO and/or MU-MIMO.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, amethod of transmitting signals by a transmitting end in a wirelesscommunication system comprises sharing control information related toreference signal with a receiving end; generating one or more precodedreference signals considering a given rank; allocating the one or moreprecoded reference signals to have a specific pattern within a subframe,wherein the specific pattern is varied depending on the controlinformation; and transmitting the subframe through multiple antennas tothe receiving end.

The control information may include information related to density oroverhead of the one or more precoded reference signals.

The control information may include information related to a frequencyinterval of the one or more precoded reference signals. The frequencyinterval may include subcarrier spacing between the one or more precodedreference signals within one subframe or slot, and the one or moreprecoded reference signals may be staggered within the subframe or slot.The frequency interval may include subcarrier spacing between the one ormore precoded reference signals within one orthogonal frequency divisionmultiplexing (OFDM) symbol. The frequency interval may be 1, 2, 3 or 4when the given rank is 1. The frequency interval may be 2, 3 or 4 whenthe given rank is 2.

The control information may include information indicating the givenrank of the scheduled data streams for the UE receiving the controlinformation.

The control information may include information indicating whether radioresources for one or more reference signals associated with other layerswhich do not correspond to scheduled data stream layers are punctured ornot.

The control information may include information indicating whether radioresources for one or more reference signals associated with other layerswhich do not correspond to scheduled data stream layers can be used fortransmitting scheduled data or not.

The control information may include information indicating whether radioresources for one or more reference signals associated with other layerswhich do not correspond to scheduled data stream layers are used by oneor more another transmitting ends.

The control information may include information indicating whether aradio resource for the subframe has been allocated to a single user ormultiple users. If the control information indicates that the radioresource for the subframe is allocated to multiple users, the one ormore precoded reference signals may be multiplexed within the subframeconsidering reference signals of other user equipments to which the sameradio resource is allocated. In this case, the control information mayfurther include information required to multiplex the one or moreprecoded reference signals with the reference signals of other userequipment.

The one or more precoded reference signals may be multiplexed using afrequency division multiplexing mode, a code division multiplexing mode,or their combination.

The one or more precoded reference signals may be cyclic-shifted in afrequency domain or a time domain.

A specific multiple access schemes may be selectively used among two ormore different multiple access schemes depending on the given rank.

The one or more precoded reference signals may be demodulation referencesignals (DMRS).

According to the embodiments of the present invention, the followingadvantages can be obtained.

First of all, it is possible to efficiently transmit reference signalsthrough multiple antennas in a wireless communication system.

Second, it is possible to flexibly control a pattern of referencesignals depending on a communication condition.

Third, it is possible to flexibly control a pattern of reference signalsto efficiently support SU-MIMO and/or MU-MIMO.

Finally, it is possible to signal for flexibly controlling a pattern ofreference signals to efficiently support SU-MIMO and/or MU-MIMO.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a schematic diagram illustrating an evolved universalterrestrial radio access network (E-UTRAN);

FIG. 2A and FIG. 2B are diagrams illustrating structures of a controlplane and a user plane of a radio interface protocol between a userequipment (UE) and E-UTRAN;

FIG. 3 is a schematic diagram illustrating antennas of a multiple inputmultiple output (MIMO) system;

FIG. 4 is a diagram illustrating a channel from transmitting antennasN_(T) to a receiving antenna i;

FIG. 5 is a block diagram illustrating a base station that can beapplied to one embodiment of the present invention;

FIG. 6 is a block diagram illustrating a user equipment that can beapplied to one embodiment of the present invention;

FIG. 7 is a block diagram illustrating a transmitter that can be appliedto one embodiment of the present invention;

FIG. 8 is a block diagram illustrating a signal generator according to asingle carrier—frequency division multiple access (SC-FDMA) scheme,which can be applied to one embodiment of the present invention;

FIG. 9 is a block diagram illustrating a receiver that can be applied toone embodiment of the present invention;

FIG. 10 is a diagram illustrating a structure of a demodulationreference signal (DMRS) of an uplink in 3GPP LTE;

FIG. 11A to FIG. 11D are diagrams illustrating examples of a precodedDMRS allocated to one OFDM symbol per slot in accordance with afrequency division multiplexing (FDM) mode;

FIG. 12A to FIG. 12D are diagrams illustrating examples of a precodedDMRS allocated to two OFDM symbols per slot in accordance with afrequency division multiplexing (FDM) mode;

FIG. 13A and FIG. 13B are flow charts illustrating a method ofuplink-transmitting reference signals when control information relatedto reference signal is received in accordance with one embodiment of thepresent invention;

FIG. 14 is a diagram illustrating an example of reference signalsallocated by a frequency division multiplexing (FDM) mode considering Mfactor in accordance with one embodiment of the present invention;

FIG. 15A to FIG. 15D are diagrams illustrating examples of referencesignals allocated with regard to each rank value by a frequency divisionmultiplexing (FDM) mode considering M factor in accordance with oneembodiment of the present invention;

FIG. 16A and FIG. 16B are diagrams illustrating another examples ofreference signals allocated with regard to each rank value by a codedivision multiplexing (CDM) mode considering M factor in accordance withone embodiment of the present invention;

FIG. 17 is a diagram illustrating an example of reference signalsallocated by a frequency division multiplexing (FDM) mode to implementMulti User (MU) MIMO in accordance with one embodiment of the presentinvention;

FIG. 18A to FIG. 18D are diagrams illustrating examples of referencesignals allocated with regard to each rank value by a frequency divisionmultiplexing (FDM) mode considering M factor in accordance with anotherembodiment of the present invention;

FIG. 19A and FIG. 19B are diagrams illustrating another examples ofreference signals allocated with regard to each rank value by a codedivision multiplexing (CDM) mode considering M factor in accordance withanother embodiment of the present invention; and

FIG. 20 is a diagram illustrating another example of reference signalsallocated by a frequency division multiplexing (FDM) mode to implementMU MIMO in accordance with another embodiment of the present invention.

FIG. 21 is a view showing a transmitter or receiver according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

E-UTRAN and Protocol Stack

FIG. 1 is a schematic diagram illustrating a network structure of anevolved universal terrestrial radio access network (E-UTRAN) which is amobile communication system to which one embodiment of the presentinvention is applied. The E-UTRAN system is a system evolving from theUTRAN system. The E-UTRAN includes base stations eNBs, which areconnected with each other through X2 interface. The eNB is connectedwith a user equipment through a radio interface, and is connected withan evolved packet core (EPC) through S1 interface.

FIG. 2A and FIG. 2B are diagrams illustrating structures of a controlplane and a user plane (U-plane) of a radio interface protocol between auser equipment (UE) and UTRAN (UMTS Terrestrial Radio Access Network)based on the 3GPP radio access network standardization. The radiointerface protocol horizontally includes a physical layer, a data linklayer and a network layer, and vertically includes a user plane for datainformation transfer and a control plane for signaling transfer. Theprotocol layers in FIG. 2A and FIG. 2B can be classified into L1 (firstlayer), L2 (second layer), and L3 (third layer) based on three lowerlayers of the open system interconnection (OSI) standard model widelyknown in the communications systems.

The control plane means a passageway where control messages aretransmitted, wherein the control messages are used in the user equipmentand the network to manage call. The user plane means a passageway wheredata generated in an application layer, for example, voice data orInternet packet data are transmitted. Hereinafter, respective layers ofthe control plane and the user plane of the radio protocol will bedescribed.

The physical layer as the first layer provides an information transferservice to an upper layer using a physical channel. The physical layer(PHY) is connected to a medium access control (hereinafter, abbreviatedas ‘MAC’) layer above the physical layer via a transport channel. Dataare transferred between the medium access control layer and the physicallayer via the transport channel. The transport channel is divided into adedicated transport channel and a common transport channel depending onchannel sharing. Also, data are transferred between different physicallayers, and more particularly, between one physical layer of atransmitting side and the other physical layer of a receiving side viathe physical channel. The physical channel is modulated in accordancewith an orthogonal frequency division multiplexing (OFDM) scheme, anduses time and frequency as radio resources.

Several layers exist in the second layer. A medium access control (MAC)layer of the second layer serves to map various logical channels withvarious transport channels. Also, the MAC layer performs multiplexingfor mapping several logical channels with one transport channel. The MAClayer is connected with a radio link control (RLC) layer correspondingto its upper layer through the logical channel. The logical channel isdivided into a control channel and a traffic channel depending on typesof transmitted information, wherein the control channel transmitsinformation of the control plane and the traffic channel transmitsinformation of the user plane.

The RLC layer of the second layer serves to perform segmentation andconcatenation of data received from its upper layer to control a size ofthe data so that the lower layer transmits the data to a radio interval.Also, the RLC layer of the second layer provides three action modes,i.e., a transparent mode (TM), an unacknowledged mode (UM), and anacknowledged mode (AM) to ensure various quality of services (QoS)required by each radio bearer (RB). In particular, the AM RLC layerperforms a retransmission function through automatic repeat and request(ARQ) for reliable data transmission.

In order to effectively transmit data using IP packets (e.g., IPv4 orIPv6) within a radio-communication interface having a narrow bandwidth,a PDCP (packet data convergence protocol) layer of the second layer (L2)performs header compression to reduce the size of IP packet headerhaving a relatively large size and unnecessary control information. Theheader compression is to increase transmission efficiency of theradio-communication interface by allowing a packet header of data totransmit necessary information only. Also, in the LTE system, the PDCPlayer performs a security function. The security function includes aciphering function preventing the third party from performing datamonitoring and an integrity protection function preventing the thirdparty from performing data manipulation.

A radio resource control (hereinafter, abbreviated as ‘RRC’) layerlocated on a lowest part of the third layer is defined in the controlplane only and is associated with configuration, re-configuration andrelease of radio bearer (hereinafter, abbreviated as ‘RB’) where the RBis in charge of controlling the logical, transport and physicalchannels. To this end, the RRC layer allows the user equipment and thenetwork to exchange RRC message with each other. If the RRC layer of theuser equipment is RRC connected with the RRC layer of the radio network,the user equipment is in RRC connected mode. If not so, the userequipment is in RRC idle mode.

In this case, the RB means a service or logical path provided by thesecond layer for the data transfer between the user equipment and theUTRAN. Generally, establishing RB means that features of a radioprotocol layer and channel required for a specific service are definedand their detailed parameters and action methods will be established.The RB is divided into a signaling RB (SRB) and a data RB (DRB). The SRBis used as a path for transmitting RRC message in a control plane, andthe DRB is used as a path for transmitting user data in a user plane.

A non-access stratum (NAS) layer located above the RRC layer performsfunctions such as session management and mobility management.

One cell constituting eNB is established at one of bandwidths of 1.25,2.5, 5, 10, and 20 Mhz and provides a downlink or uplink transmissionservice to several user equipments. At this time, different cells can beestablished to provide different bandwidths.

As downlink transport channels carrying data from the network to theuser equipment, there are provided a broadcast channel (BCH) carryingsystem information, a paging channel (PCH) carrying paging message, anda downlink shared channel (SCH) carrying user traffic or controlmessages. The traffic or control messages of a downlink multicast orbroadcast service can be transmitted via the downlink SCH or anadditional downlink multicast channel (MCH). Meanwhile, as uplinktransport channels carrying data from the user equipments to thenetwork, there are provided a random access channel (RACH) carrying aninitial control message and an uplink shared channel (UL-SCH) carryinguser traffic or control message.

As logical channels located above the transport channels and mapped withthe transport channels, there are provided a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

Modeling of MIMO System

FIG. 3 is a schematic diagram illustrating a wireless communicationsystem having multiple antennas. As illustrated in FIG. 3, if the numberof transmitting antennas is increased to N_(T) and the number ofreceiving antennas is increased to N_(R), theoretical channeltransmission capacity is increased in proportional to the number ofantennas unlike the case that a plurality of antennas are used only ineither a transmitter or a receiver. Accordingly, it is possible toimprove a transmission rate and remarkably improve frequency efficiency.As channel transmission capacity increases, the transmission rate cantheoretically increase as much as a value obtained by multiplying amaximum transmission rate R_(o) by a rate increase rate R_(i).R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, in a MIMO communication system that uses four transmittingantennas and four receiving antennas, a transmission rate of four timesmore than that of a single antenna system can be obtained. Varioustechnologies for improving a substantial data transmission rate havebeen studied after the theoretical capacity of a MIMO system has provedin the mid 90's. Also, several technologies have been reflected upon thestandard of various wireless communications such as 3rd generationmobile communication and next generation wireless LAN.

Upon review of the trend of recent studies related to multiple antennas,it is noted that active studies are ongoing in view of various aspectssuch as information theoretical studies related to calculation ofmulti-antenna communication capacity under various channel environmentsand multiple access environments, studies for measuring a radio channeland forming a pattern in a MIMO system, and studies of the space timesignal processing technology for improving transmission reliability anda transmission rate.

A communication method in a MIMO system will be described in more detailwith reference to mathematical modeling. It is assumed that N_(T) numberof transmitting antennas and N_(R) number of receiving antennas exist inthe MIMO system.

Upon review of a transmitting signal, when N_(T) number of transmittingantennas exist, the number of maximum available information fortransmission is N_(T). The transmission information can be expressed asfollows.s=└s ₀ ,s ₂ , . . . ,s _(N) _(T) ┘^(T)

Each transmission information s₁, s₂, . . . , s_(N) _(T) can have itsown transmission power. Supposing that each transmission power is P₁,P₂, . . . , P_(N) _(T) , transmission information of which transmissionpower has been controlled can be expressed as follows.ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

Also, Ŝ can be expressed as follows using a diagonal matrix P of thetransmission power.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\;\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {P\; s}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

It is considered that N_(T) number of transmitting signals x₁, x₂, . . ., x_(N) _(T) are generated as a weight matrix W is applied to theinformation vector ŝ of which transmission power has been controlled. Inthis case, the weight matrix W serves to appropriately distributetransmission information to each antenna depending on a transportchannel status. x₁, x₂, . . . , x_(N) _(T) can be expressed as followsusing a vector X.

$\begin{matrix}{x = {{{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix}\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1\; N_{T}} \\w_{21} & w_{22} & \ldots & w_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{i\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {W\; P\; s}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

In this case, w_(ij) means a weight value between the i-th transmittingantenna and the j-th information. W is also referred to as a precodingmatrix.

When N_(R) number of receiving antennas exist, receiving signals y₁, y₂,. . . , y_(N) _(R) of each antenna can be expressed as follows using avector.y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

When channel modeling is performed in the multi-antenna wirelesscommunication system, the channel can be classified depending ontransmitting and receiving antenna indexes. A channel that passes fromthe transmitting antenna j to the receiving antenna i will be expressedas h_(ij). It is noted that the receiving antenna index is prior to thetransmitting antenna index.

FIG. 4 shows channels from N_(T) number of transmitting antennas to thereceiving antenna i. The channels can be expressed in a vector andmatrix type. In FIG. 4, the channels arrived from a total of N_(T)number of transmitting antennas to the receiving antenna i can beexpressed as follows.h _(i) ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

Accordingly, all channels arrived from N_(T) number of transmittingantennas to N_(R) number of receiving antennas can be expressed asfollows.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{i\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Additive white Gaussian noise (AWGN) is added to a real channel afterpassing through a channel matrix H. AWGN n₁, n₂, . . . , n_(N) _(R)added to each of the N_(R) number of receiving antennas can be expressedas follows.n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

The receiving signals can be expressed as follows through theaforementioned equation modeling.

                                     [Equation  10]$y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1\; N_{T}} \\h_{21} & h_{22} & \ldots & h_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{i\; N_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{H\; x} + n}}}$

The aforementioned description is based on that the multi-antennacommunication system is applied to a single user. However, themulti-antenna communication system may be applied to a plurality ofusers, whereby multiuser diversity can be obtained. This will bedescribed as follows.

A fading channel is well known as a main factor that deterioratesthroughput of the wireless communication system. Channel gain is varieddepending on time, frequency and space, and throughput is remarkablydeteriorated as a channel gain value is low. Diversity which is one ofmethods that can resolve fading uses the fact that it is not likely thatseveral independent channels all have low gain. Various diversityschemes can be provided, and an example of the various diversity schemesincludes multiuser diversity.

When several users are within a cell, channel gain of each user isindependent in view of probability. Accordingly, it is not likely thatchannel gains of the users are all low.

According to the information theory, when the transmission power of thebase station is sufficient and several users are within a cell, allchannels may be allocated to a user having the highest channel gain,whereby total capacity of the channels can be maximized. Multiuserdiversity can be divided into three types as follows.

First of all, temporal multiuser diversity is to allocate a channel to auser having the highest gain value when the channel is varied dependingon time. Frequency multiuser diversity is to allocate a subcarrier to auser having the highest gain value in each frequency domain of afrequency multiplexing carrier system such as Orthogonal FrequencyDivision Multiplexing (OFDM).

If a channel is very slowly changed in a system that does not usemultiple carriers, the user having the highest channel gain value willexclusively occupy the channel for a long time. For this reason, otherusers cannot perform communication. In this case, for application ofmultiuser diversity, it is necessary to guide channel change.

Next, according to spatial multiuser diversity, channel gains of usersare different from one another depending on space. An example of thespatial multiuser diversity includes random beamforming (RBF). The RBFis also referred to as “opportunistic beamforming.” According to theRBF, the transmitter performs beamforming at a random weight value usingmultiple antennas to guide channel change.

A multiuser MIMO (MU-MIMO) scheme that uses the aforementioned multiuserdiversity for a MIMO scheme will be described as follows.

According to the multiuser MIMO scheme, the transmitter and the receiverenable several kinds of combinations in the number of users and thenumber of antennas of each user. The multiuser MIMO scheme will bedescribed based on a downlink (forward link) and an uplink (reverselink). The downlink means a communication link through which the basestation transmits a signal to a user equipment. The uplink means acommunication link through which a user equipment transmits a signal tothe base station.

In case of the downlink, for extreme examples, one user may receive asignal through a total of N_(R) antennas, or a total of N_(R) users mayrespectively receive a signal through one antenna. Also, intermediatecombination of the extreme examples may be provided. Namely, combinationcan be provided in such a manner that a user uses one receiving antennawhereas another user uses three receiving antennas. It is noted that thenumber of a total of receiving antennas is equally maintained as N_(R)in any case. This case will be referred to as MIMO BC (BroadcastChannel) or SDMA (Space Division Multiple Access).

In case of the uplink, for extreme examples, one user may transmit asignal through a total of N_(T) antennas, or a total of N_(T) users mayrespectively transmit a signal through one antenna. Also, intermediatecombination of the extreme examples may be provided. Namely, combinationcan be provided in such a manner that a user uses one transmittingantenna whereas another user uses three transmitting antennas. It isnoted that the number of a total of transmitting antennas is equallymaintained as N_(T) in any case. This case will be referred to as MIMOMAC (Multiple Access Channel). Since the uplink and the downlink are ina symmetrical relation to each other, the scheme used in any one side ofthe uplink and the downlink may be used in the other side.

Meanwhile, the number of rows and columns of the channel matrix Hrepresenting the channel status is determined by the number oftransmitting and receiving antennas. In the channel matrix H, the numberof rows is equal to the number N_(R) of receiving antennas, and thenumber of columns is equal to the number N_(T) of transmitting antennas.Namely, the channel matrix H becomes N_(R)×N_(T).

A rank of the matrix is defined by a minimum number of rows or columnsindependent from one another. Accordingly, the rank of the matrix cannotbe greater than the number of rows or columns. The rank rank(H) of thechannel matrix H is limited as follows.rank(H)≦min(N _(T) ,N _(R))  [Equation 11]

Alternatively, the rank can be defined by the number of eigenvalues not0 when eigenvalue decomposition is performed for the matrix. Similarly,the rank can be defined by the number of singular values not 0 whensingular value decomposition is performed for the matrix. Accordingly,in the channel matrix, the rank can physically be defined as a maximumnumber that can be transmitted from a given channel as different kindsof information.

Each of different information transmitted using multiple antenna schemesmay be defined “transmitting stream” or more simply “stream” unlesscontrary defined. The term stream may be also referred to as “layer.”Then, the number of transmitting stream (or layer) cannot be larger thanthe rank of channel, which is the maximum number of different kinds ofinformation to be transmitted through the channel.

Transmitter and Receiver of MIMO System

FIG. 5 is a block diagram illustrating a base station that can beapplied to one embodiment of the present invention.

Referring to FIG. 5, the base station includes a control system 502, abaseband processor 504, a transmission circuit 506, a receiving circuit508, multiple antennas 510, and a network interface 512. The receivingcircuit 508 receives a radio signal from an user equipment through themultiple antennas 510. Preferably, a low noise amplifier and filter (notshown) amplifies a signal and filters broadband interference. Adown-conversion and digitalization circuit (not shown) down-converts thefiltered receiving signal to an intermediate or baseband frequencysignal and digitalizes the down-converted signal to one or more digitalstreams.

The baseband processor 504 processes the digitalized receiving signaland extracts information or data bits from the receiving signal. In thiscase, the processing includes demodulation, decoding, and errorcorrection. The baseband processor 504 is generally implemented by oneor more digital signal processors (DSP). Afterwards, receivinginformation is transmitted to a radio network through the networkinterface, or is transmitted to another user equipment served by thebase station. The network interface 512 interacts with a circuit lineexchange network that forms a part of a radio network that can beconnected with a central network controller and a public switchedtelephone network (PSTN).

At the transmitter, the baseband processor 504 receives digitalized datathat can represent voice, data or control information, from the networkinterface 512 under the control of the control system 502, and encodesthe data for transmission. The encoded data are input to thetransmission circuit 506. The encoded data are modulated in thetransmission circuit 506 by desired transmission frequencies or carriershaving frequencies. A power amplifier (not shown) amplifies themodulated carrier signal at a proper level. The amplified signal istransmitted to the multiple antennas 510.

FIG. 6 is a block diagram illustrating a user equipment that can beapplied to one embodiment of the present invention.

Referring to FIG. 6, the user equipment includes a control system 602, abaseband processor 604, a transmission circuit 606, a receiving circuit608, multiple antennas 610, and a user interface 612. The receivingcircuit 608 receives a radio signal including information from one ormore base stations through the multiple antennas 610. Preferably, a lownoise amplifier and filter (not shown) amplifies a signal and filtersbroadband interference. Afterwards, a down-conversion and digitalizationcircuit (not shown) down-converts the filtered receiving signal to anintermediate or baseband frequency signal and then digitalizes thedown-converted signal to one or more digital streams. The basebandprocessor 604 processes the digitalized receiving signal to extractinformation or data bits from the receiving signal. In this case, theprocessing includes demodulation, decoding, and error correction. Thebaseband processor 604 is generally implemented by one or more digitalsignal processors (DSP) and an application-specific integrated circuit(ASIC).

At the transmitter, the baseband processor 604 receives digitalized datathat can represent voice, data or control information, from the userinterface 612 under the control of the control system 602, and encodesthe data for transmission. The encoded data are input to thetransmission circuit 606. The encoded data are modulated in thetransmission circuit 606 by desired transmission frequencies or carriershaving frequencies. A power amplifier (not shown) amplifies themodulated carrier signal at a proper level. The amplified signal istransmitted to the multiple antennas 610.

FIG. 7 is a block diagram illustrating a transmitter that can be appliedto one embodiment of the present invention.

Referring to FIG. 7, although the transmitter has been described basedon the base station, the person with ordinary skill in the art will knowthat the shown structure can be used for uplink and downlinktransmission. Also, the transmission structure is intended to representvarious multiplexing access structures that include, but not limited to,code division multiple access (CDMA), frequency division multiple access(FDMA), time division multiple access (TDMA), and orthogonal divisionmultiple access (OFDM).

Initially, the network transmits data, which are intended to betransmitted to the user equipment, to the base station. Bit streams,i.e., scheduled data are scrambled by a data scramble module 704 toreduce a peak to average power ratio associated with data. Cyclicredundancy check (CRC) for the scrambled data is determined by a CRCadding module 706 and added to the scrambled data. The user equipmentperforms channel coding using a channel encoder module 708 to facilitatedata recovery and error correction. Redundancy can effectively be addedto the data by channel coding. The channel encoder module 708 can use aturbo encoding technology.

The processed data bits are mapped into corresponding symbols by amapping module 714 depending on the selected baseband modulation. Inthis case, quadrature amplitude modulation (QAM) or quadrature phaseshift keying (QPSK) modulation can be used. Bit groups are mapped intosymbols representing a location in a phase constellation and amplitude.Afterwards, symbol blocks are processed by a space time code (STC)encoder module 718. The STC encoder module 718 processes symbols inaccordance with a selected STC encoding mode and provides N number ofoutputs corresponding to the number of multiple transmitting antennas510 of the base station. The symbol streams output from the STC encodermodule 718 are inverse-fourier transformed by an IFFT processing module720. Afterwards, a prefix and RS insertion module 722 inserts cyclicprefix (CP) and RS to the inverse-fourier transformed signal.Afterwards, a digital uplink conversion (DUC) and digital to analog(D/A) conversion module 724 uplink converts the signal processed by theprefix and RS insertion module 722 into an intermediate frequency in adigital region and converts the processed signal into an analog signal.Then, the analog signal is modulated, amplified and transmitted at adesired RF frequency through the RF module 726 and the multiple antennas510.

FIG. 8 is a block diagram illustrating a signal generator according to asingle carrier—frequency division multiple access (SC-FDMA) scheme,which can be applied to one embodiment of the present invention.

Referring to FIG. 8, the signal generator includes a discrete fouriertransform (DFT) module 810 performing DFT, a subcarrier mapper 820, andan inverse fast fourier transform (IFFT) module 830 performing IFFT. TheDFT module 810 performs DFT for input data and then outputs frequencydomain symbols. The subcarrier mapper 820 maps the frequency domainsymbols into each subcarrier, and the IFFT module 830 performs IFFT forthe input symbols to output a time domain signal.

FIG. 9 is a block diagram illustrating a receiver that can be applied toone embodiment of the present invention.

Although the receiver is described based on the user equipment in FIG.9, the person with ordinary skill in the art will know that the shownstructure can be used for uplink and downlink transmission. If transportsignals reach the multiple transmitting antennas 610, each signal isdemodulated and amplified by an RF module 902. For convenience, one ofmultiple receiving paths is shown. An analog to digital (A/D) conversionand downlink conversion module (DCC) module 904 digitalizes an analogsignal for digital processing and downlink-converts the signal. Thedigitalized signal can be used for an automatic gain control (AGC)module 906 to control amplifier gain from the RF module 902 based on thereceived signal level.

The digitalized signal is also provided to a synchronization (Sync)module 908. The synchronization module 908 includes a coarse Sync module910 performing brief synchronization, a fine Sync module 912 performingfine synchronization, and a module 920 estimating frequency offset andDoppler effect. The result output from the Sync module 908 is providedto a frame alignment module 914 and a frequency offset/Dopplercorrection module 918. A CP is removed from the aligned frame by aprefix removal module 916. Afterwards, the data from which CP is removedare fourier-transformed by an FFT module 922. An RS extract module 930extracts RS signal scattered within a frame and provides the extractedRS signal to a channel estimation module 928. Then, a channelreconstruction module 926 reconstructs a radio channel using the resultof channel estimation. The channel estimation provides sufficientchannel response information so that a STC decoder 932 can decodesymbols in accordance with STC encoding used by the base station andrecover estimation corresponding to transmission bits. Symbols obtainedfrom the received signal and the result of channel estimation for eachreceiving path are provided to the STC decoder 932, and the STC decoder932 performs STC decoding for each receiving path to recover thetransmitted symbols. The STC decoder 932 can perform maximum likelihooddecoding (MLD) for BLAST based transmission. The output of the STCdecoder 932 could be a log likelihood ratio (LLR) for each oftransmission bits. The STC decoded symbols are aligned as those of theoriginal order through a symbol de-interleaver module 934. Afterwards, ade-mapping module 936 and a bit de-interleaver module 938 map symbolsinto bit streams and perform de-interleaving. The bit streams processedby a rate de-matching module 940 are provided to a channel decodermodule 942 to recover the scrambled data and CRC checksum. The channeldecoder module 942 can use turbo decoding. The CRC module 944 removesCRC checksum in accordance with a conventional manner and checks thescrambled data. Afterwards, the CRC checked data are recovered tooriginal data 948 by a descrambling module 946.

Reference Signal (RS)

When packets are transmitted in a wireless communication system, sincethe packets are transmitted through a radio channel, signal distortionmay occur during packet transmission. In order to receive the distortedsignal normally, the receiver should correct distortion of the receivedsignal using channel information. In order to identify channelinformation, a signal known by both the transmitter and the receiver istransmitted so that the channel information is identified by adistortion level at the time when the signal is received through achannel. In this case, the signal is referred to as a pilot signal or areference signal. When data are transmitted and received throughmultiple antennas, a channel status between each transmitting antennaand each receiving antenna should be identified to receive a signalnormally. Accordingly, a separate reference signal is required for eachtransmitting antenna.

Code Division Multiplexing (CDM)—Cyclic Delay in Time Domain

Multiplexing means that reference signals (RS) for different antennasare allocated to one resource region. Examples of the multiplexinginclude time division multiplexing, frequency division multiplexing andcode division multiplexing. Of them, the code division multiplexingmeans that different orthogonal codes (sequences) separately set foreach antenna are multiplied by RS in a frequency domain to allocate themultiplied results to one radio resource (frequency/time). Theorthogonal codes could be a type such as

${\mathbb{e}}^{j\; 2\;\pi\frac{u}{M}{\mathbb{i}}}.$As the orthogonal codes are multiplied in the frequency domain, the RSmay be cyclic delayed in a time domain. If the orthogonal codes aremultiplied by a sequence x[i]={x₀, x₁, x₂, . . . , x_(N-1)}, cyclicdelay in the time domain is expressed as follows.

$\begin{matrix}{{{\begin{matrix}{{{X^{\prime}\left\lbrack k^{\prime} \right\rbrack} = {\sum\limits_{i = 0}^{N - 1}{{x\lbrack i\rbrack} \cdot {\mathbb{e}}^{j\; 2\;\pi\frac{k^{\prime}}{N}{\mathbb{i}}}}}},{N = {M\; Z}},{k^{\prime} = 0},1,\ldots\mspace{14mu},{N - 1}} \\{= {\sum\limits_{i = 0}^{N - 1}{{x\lbrack i\rbrack} \cdot {\mathbb{e}}^{j\; 2\;{\pi{(\frac{k^{\prime} + {u\; Z}}{N})}}{\mathbb{i}}}}}}\end{matrix}\mspace{20mu}{X^{\prime}\left\lbrack {\left( {k - {u\; Z}} \right){mod}\; N} \right\rbrack}} = {\sum\limits_{i = 0}^{N - 1}{{x\lbrack i\rbrack} \cdot {\mathbb{e}}^{j\; 2\;\pi\frac{k}{N}{\mathbb{i}}}}}}\mspace{20mu}{{X^{\prime}\left\lbrack {\left( {k - {u\; Z}} \right){mod}\; N} \right\rbrack} = {X\lbrack k\rbrack}}} & \left\lbrack {{Equation}\mspace{14mu} 16} \right\rbrack\end{matrix}$

Accordingly, if

${\mathbb{e}}^{j\; 2\;\pi\frac{u}{M}{\mathbb{i}}}$is multiplied in the frequency domain, cyclic delay occurs on the timeaxis.

Structure of Uplink RS in the Conventional 3GPP LTE

FIG. 10 is a diagram illustrating a structure of uplink referencesignals (RS) in the conventional 3GPP LTE.

Referring to FIG. 10, a radio frame includes ten (10) subframes, each ofwhich includes two slots. A transmission time interval (TTI) means thetime required to transmit one subframe. In the 3GPP LTE, a subframe is 1ms, and a slot is 0.5 ms. However, the structure of the radio frame andTTI can be varied depending on a communication system. The structure ofthe radio frame and subframe can be also used in downlink.

A slot includes a plurality of SC-FDMA symbols in a time domain, and aplurality of resource blocks in a frequency domain. For one resourceblock, a horizontal axis means a time axis, and a vertical axis means afrequency axis. In case of normal CP, each slot may include sevensymbols. In case of extended CP, each slot may include six symbols. Theextended CP is generally used under the environment where delay is long.The RS uses all resources of one symbol to satisfy single carriercharacteristics in a single carrier-frequency division multiple access(SC-FDMA) system. Meanwhile, unlike data, DFT transformation precodingis not applied to the RS in the uplink, and the RS includes demodulationRS (DMRS) and sounding RS (SRS). In FIG. 10, the DMRS are allocated tol=4 of slots 1 and 2 and expressed as ‘1.’ The SRS are allocated to l=6of slot 2. The data are allocated to the other resource elements.

Transmission of DMRS Using Multiple Antennas

It is noted that the following embodiment of the present invention isdescribed based on the LTE-A but can equally be applied to all MIMOsystems. Moreover, it is also noted that the following embodiment of thepresent invention is described based on MIMO transmission in uplink forsimplicity but the exemplified embodiments can also applied to MIMOtransmission in downlink. This can be easily understood and achievedsince the uplink and the downlink are in a symmetrical relation to eachother, the scheme used in any one side of the uplink and the downlinkmay be used in the other side.

In the 3GPP LTE system, the user equipment supports a single antennaonly. Accordingly, in the LTE system, the number of antennas used forthe uplink is one. However, in case of the LTE-A system, the userequipment supports MIMO even in the uplink. Accordingly, the DMRS shouldbe extended properly considering MIMO scheme. In order to extend theDMRS to conform to the MIMO environment, non-precoded DMRS and precodedDMRS can be considered. In the same manner as the downlink according tothe related art, the non-precoded DMRS needs DMRS patterns equivalent tothe number of antennas. Namely, in case of the non-precoded DMRS, DMRSpatterns should be defined depending on the number of antennas that canbe supported by the system. However, in case of the precoded-DMRS, aprecoding matrix is multiplied by channel information measured by theUser and DMRS patterns are applied for each rank corresponding to avirtual antenna domain. Accordingly, it is advantageous in that overheadof DMRS can be reduced even though the number of antennas increases. Inother words, in case of the precoded DMRS, DMRS patterns should bedefined depending on the rank that can be supported by the differentchannel conditions. For example, supposing that the number oftransmitting antennas, which can be supported by the system, is 1, 2 and4, three patterns should be defined in the non-precoded DMRS whilepatterns should be defined for rank 1, 2, 3 and 4 in the precoded DMRS.Hereinafter, the case where the number of transmitting antennas is 4will be described. However, the following embodiment of the presentinvention can similarly be applied to all systems having a plurality oftransmitting antennas. Although the precoded DMRS and the non-precodedDMRS should be applied to the LTE-A system considering their advantagesand disadvantages, since the precoded DMRS covers all available ranks,DMRS pattern for each rank will be defined. However, the non-precodedDMRS may have the same pattern as that of the precoded DMRS in otherranks except for rank 3.

Examples of Allocation Pattern of Reference Signal According to Rank

FIG. 11 illustrates examples of a precoded DMRS allocated to one OFDMsymbol per slot in accordance with a frequency division multiplexing(FDM) mode.

FIG. 11A to FIG. 11D illustrate structures of DMRS when rank of a userequipment is 1 to 4. As described above, in case of the non-precodedDMRS, DMRS patterns of rank 1, rank 2 and rank 4 can be used as those ofone antenna, two antennas and four antennas. In FIG. 11A to FIG. 11D,each subframe is used for transmitting each corresponding layer.Alternatively, each subframe can be assumed to be transmitted througheach corresponding virtual antenna. In FIG. 11A to FIG. 11D, ‘1’, ‘2’,‘3’ and ‘4’ respectively mean DMRS for layer 1 (or virtual antenna 1),layer 2 (or virtual antenna 2), layer 3 (or virtual antenna 3), andlayer 4 (or virtual antenna 4). Parts marked with ▪ in each subframerepresent resource elements related with other reference signals.Depending on communication conditions, MIMO environments, MIMO schemeand the like, in each subframe, resource elements related with otherreference signals may be used to transmit data. Alternatively, in eachsubframe, resource elements related with other reference signals may bepunctured to provide orthogonality between DMRS for layers in SU-MIMO orbetween DMRS for different users in MU-MIMO in terms of totaltransmission rank. Namely, other data or DMRS for other rank may betransmitted to the parts marked with ▪. In FIG. 11, DMRS is transmittedto one symbol per slot. However, in the MIMO-OFDM system, DMRS can betransmitted to several symbols.

FIG. 12A to FIG. 12D are diagrams illustrating examples of a precodedDMRS allocated to two OFDM symbols per slot in accordance with afrequency division multiplexing (FDM) mode. FIG. 12 is basically thesame as FIG. 11 except that the reference signals are allocated usingtwo OFDM symbols per slot. Accordingly, for the description of FIG. 12,refer to the description of FIG. 11.

Embodiment Dynamic Control of Pattern of Reference Signal

According to the related art, even though a rank value is varied asillustrated in FIG. 11 and FIG. 12, basic pattern for allocating areference signal of each rank has not been changed. However, as acommunication environment is changed, it is required to flexibly controla pattern of reference signals. For example, a pattern of requiredreference signals may be varied depending on quality of service (QoS),channel delay, MIMO mode (SU-MIMO, MU-MIMO), etc. Accordingly, a methodof flexibly controlling a pattern of reference signals depending on acommunication environment is required. In this respect, the presentinvention suggests a signaling method for flexibly controlling a patternof reference signals. Only, flexible pattern of reference signal maycause inefficient use of data resources or deteriate orthogonalitybetween reference signals. Thus the present invention further suggests asignaling method for efficiently use resources related with the flexiblepattern of reference signals. Hereinafter, a MIMO environment based onOFDM will be described unless additionally specified. Also, as can beseen from FIG. 10, in the conventional 3GPP LTE system, overhead of DMRSfor uplink transmission is 14.3%. Accordingly, the present inventionsuggests a method of transmitting DMRS of which overhead does not exceed14.3% even though a single antenna system is extended to a MIMO system.

FIG. 13A is a flow chart illustrating a method of uplink-transmittingreference signals when control information related to reference signalis received in accordance with one embodiment of the present invention.It is noted that the present embodiment can be applied todownlink-transmitting reference signals since the uplink and thedownlink are in a symmetrical relation to each other, the scheme used inany one side of the uplink and the downlink may be used in the otherside. In that case, signaling for the control information related toreference signal can be performed in a same or similar manner.

Referring to FIG. 13A, the user equipment receives control informationrelated to reference signal (CIRS) from the base station (S1310). Thecontrol information includes all kinds of information related toallocation of reference signals. For example, the control informationincludes information for controlling a pattern of reference signals,information related to multiplexing, information related to whetherresources associated with reference signals of other ranks, layers orusers are used to transmit data or punctured to provide orthogonality,etc. Afterwards, the user equipment generates a precoded referencesignal considering a rank value for uplink transmission. The referencesignals could be DMRS. The user equipment allocates the precodedreference signal to a subframe to have a specific pattern depending onthe control information (S1330). Then, the user equipment transmits thesubframe to which reference signals are allocated with a specificpattern, to the base station (S1340). In this case, the specific patternis used as a broad concept that includes a method (for example,multiplexing method) used to allocate reference signals, as well ascombination of specific locations where the reference signals arearranged within a resource block or a subframe, unless otherwisespecified.

Next, a method of signaling the control information related to referencesignal in a user equipment will be described. Details of the controlinformation and allocation of reference signals depending on the controlinformation will be described later with reference to the drawings.

The control information related to reference signal can be transmittedto the user equipment through system information (SI), RRC message,L1/L2 control signaling (for example, PDCCH) or MAC/RLC/PDCP PDU. TheRRC signal could be a signal related to RRC connection release, RRCconnection request, RRC connection setup, radio bearer setup, radiobearer re-setup, and RRC connection re-establishment.

L1/L2 control signaling is located in the first n OFDM symbols where nmay be equal to or less than 4. L1/L2 control signaling may betransmitted through physical downlink control channel (PDCCH). PDCCHcarries various downlink control information using various downlinkcontrol information (DCI) format. Multiple PDCCHs are supported and auser equipment may monitor a set of control channels. PDCCHs are formedby aggregation of control channel elements (CCEs). Each CCE includes 9resource element groups (REGs) and each REG consists of 4 resourceelement.

The control information related to reference signal may be explicitlysignaled or implicitly signaled with regard to L1/L2 control signaling.As an example of explicit signalling, PDCCH may carry informationindicating the control information related to reference signal. PDCCHmay carry the control information related to reference signal in variousformat such as bitmap, index and the like. Just for example, if bitmapis used, each bit may represent pattern of reference signals with regardto rank (e.g. pattern of reference signal for corresponding layer), andextra one or more bits may represent whether resource associated withreference signals of other ranks, layers or users will be used fortransmitting data or will be punctured for providing orthogonalitybetween layers in SU-MIMO or between users in MU-MIMO. Indexing may bealso designed in a similar manner.

As an example of implicit signalling, the control information related toreference signal may be identified by using specific structure orformation of PDCCH. A user equipment may identify the controlinformation related to reference signal from a certain CCE index relatedwith corresponding PDCCH, the number of scheduled resources,identity/difference of scheduled resources, and the like.

The control information could be user equipment-common (UE-common)information or user equipment-specific (UE-specific) information. If thecontrol information is UE-common information, the control information iscommon for a unit of PLMN, registered area, tracking area (TA), cell,group or RAT. For example, the control information can be transmitted toall user equipments within a cell through system information. Also, thecontrol information is transmitted through release of RRC connection, sothat only a specific user equipment performs the operation according tothe embodiment of the present invention. In other words, a method oftransmitting the control information to the user equipment and anapplication range with regard to the user equipment can be varieddepending on whether the control information is UE-common information orUE-specific information.

The control information can be indicated by the base stationperiodically/non-periodically. Also, the control information can beinvalided in some cases. For example, when the control information isUE-common information, the control information can be invalidated ifPLMN, tracking area (TA), cell, group or RAT is changed. For anotherexample, when the control information is UE-specific information, thecontrol information can be invalidated as the user equipment is shiftedfrom an idle mode to a connection mode. Namely, the control informationcan be invalidated by a specific RRC signal for shifting the userequipment from the idle mode to the connection mode. For example, thecontrol information can be invalidated at the time when the userequipment sends RRC connection request, receives RRC connection setupfrom the base station, or sends RRC connection complete to the basestation. For example, the control information can be invalidated by RRCconnection. Also, the user equipment can invalidate the controlinformation when a predetermined time passes after receiving the controlinformation. On the other hand, the control information can be invalidedas the user equipment is shifted from the connection mode to the idlemode. Namely, the control information can be invalidated by a specificRRC signal for shifting the user equipment from the connection mode tothe idle mode.

FIG. 13B is a flow chart illustrating a method of uplink-transmittingreference signals when control information related to reference signalis repeatedly received in accordance with one embodiment of the presentinvention. It is noted that the present embodiment can be applied todownlink-transmitting reference signals since the uplink and thedownlink are in a symmetrical relation to each other, the scheme used inany one side of the uplink and the downlink may be used in the otherside. In that case, signaling for the control information related toreference signal can be performed in a same or similar manner.

The user equipment can repeatedly receive control information from thebase station. In this case, the same control information or differentkinds of control information may be transmitted to the user equipment.If the user equipment repeatedly receives the control information, theuser equipment can apply the UE-specific control information prior tothe UE-common control information. Also, the user equipment can applycontrol information received by a specific method, prior to controlinformation received by other methods. For example, after receivingcontrol information through system information (S1310), the userequipment may receive control information once more through RRC message(for example, RRC connection) (S1312). In this case, the user equipmentdisregards control information CIRS_1 received from the systeminformation and uplink-transmits a signal in accordance with controlinformation CIRS_2 received through the RRC message (S1330, S1340).

Hereinafter, the control information related to reference signal will bedescribed in detail.

In structure of an uplink DMRS according to the 3GPP LTE system, DMRS istransmitted through one symbol as illustrated in FIG. 10. However, theLTE-A system may not be required to transmit DMRS using all resourceelements in one symbol, e.g. 12 consecutive resource elementsdependingon employed multiple access scheme. This is preferable in view ofoverhead.

Accordingly, the control information can include information related tooverhead or density of the reference signals. Overhead of the referencesignals means a ratio occupied by a resource element to which referencesignals are allocated, among all resource elements included in aresource block. Density of the reference signals can mean a ratiooccupied by a resource element to which reference signals are allocated,among a total of resource elements included in a resource block. Thedensity can represent density of the reference signals within a specificregion. The specific region could be a unit of subframe, slot and OFDMsymbol. Accordingly, density of the reference signals may be differenteven in case of the same overhead. The overhead and the density can bedetermined considering all reference signals. Preferably, the density orthe overhead can be defined based on a reference signal of each rank.

Furthermore, the control information can include information related toa frequency interval of the reference signal of each rank. In this case,the frequency interval means a subcarrier interval, e.g. the number ofsubcarrier, between adjacent (or neighboring) reference signals on afrequency axis. At this time, the adjacent reference signals may not bewithin a single OFDM symbol, and may be distributed into several OFDMsymbols, slots, or subframes. The frequency difference betweensubcarriers can be varied depending on a profile of the OFDM system. Forexample, the frequency spacing between subcarriers could be 15 kHz. Forconvenience, information related to the frequency interval will bereferred to as ‘M factor.’ The ‘M factor’ may mean an interval betweenDMRS of the same rank. The M factor can be defined by the base stationconsidering a rank value of the user equipment. If the user equipmentindicates the M factor, an allocation pattern of the reference signal inthe user equipment can be determined. If an empty space (resourceelement) occurs between the DMRS due to the M factor, the user equipmentcan transmit data using the empty space or vacate the space for DMRS ofother rank, layer or users. Information as to how the space can be usedcan be included in the control information, can be signaled separately,or can be predetermined.

FIG. 14 is a diagram illustrating an example of reference signalsallocated to one OFDM symbol per slot by a frequency divisionmultiplexing (FDM) mode considering M factor in accordance with oneembodiment of the present invention.

Referring to FIG. 14, when a rank value is 1, M could be 1, 2, 3 or 4.Also, when a rank value is 2, M could be 2, 3 or 4. Also, when a rankvalue is 3, M could be 3 or 4. Also, when a rank value is 4, the numberof DMRS becomes insufficient if a resource for data is empty.Accordingly, when a rank value is 4, one symbol is used by the DMRSonly. Since a location of symbols illustrated in FIG. 14 is exemplary,the reference signals can be allocated to other symbols. Also, althoughthe reference signals are allocated to one OFDM symbol per slot in FIG.14, the reference signals may be allocated to two or more OFDM symbols.Also, since the location of the DMRS arranged by the M factor isexemplary, the DMRS may be arranged in another manner if the frequencyinterval of the DMRS is assured. Unlike the example of FIG. 14, thefrequency interval according to the M factor can be defined as frequencyinterval between neighboring reference signals within one subframe orslot. In this case, the DMRS can be staggered within the subframe orslot.

FIG. 15 is a diagram illustrating an example of reference signalsallocated with regard to each rank considering M factor in accordancewith one embodiment of the present invention. In FIG. 15, referencesignals are allocated to one OFDM symbol per slot. FIG. 15A to FIG. 15Dillustrate examples of allocating reference signals when a rank value is1 to 4. In this case, a frequency interval of the reference signals isdefined as frequency interval between neighboring reference signalswithin one OFDM symbol.

Referring to FIG. 15A, since a rank value is 1, the M factor could be 1,2, 3 or 4. It is noted that density, overhead or frequency interval ofthe reference signals is varied depending on a value of the M factor.When the M factor is 2, 3, or 4, an empty resource element occursbetween the reference signals. As described above, the empty resourceelement can be used for data transmission, or can be punctured. When theM factor is 2, 3, or 4, the reference signals are located in differentsubcarriers of slot 1 and slot 2 of the subframe, and are cyclic-shiftedin a frequency domain to obtain frequency diversity. In addition, thereference signals can be cyclic-shifted in a time domain. Informationrequired to cyclic-shift the reference signals in the frequency or timedomain can be included in the control information related to referencesignal, or can be signaled separately. The information required tocyclic-shift the reference signals could be frequency offset or timeoffset. When M is 4, the reference signals of slot 1 and slot 2respectively have frequency offset of 0 and 2 based on subcarrier of 0(SC=0).

FIG. 15B to FIG. 15D are basically similar to FIG. 15A except that arank value is different. Accordingly, for description of FIG. 15B toFIG. 15D, refer to the description of FIG. 15A.

FIG. 16A and FIG. 16B are diagrams illustrating another examples ofreference signals allocated with regard to each rank by a code divisionmultiplexing (CDM) mode considering a factor M in accordance with oneembodiment of the present invention.

Referring to FIG. 16A, a rank value is 2 and 4. When a rank value is 2,reference signals of layer 1 and layer 2 should be transmitted.Likewise, when a rank value is 4, reference signals of layers 1 to 4should be transmitted. In each case, virtual antennas corresponding toeach rank are paired and then multiplexed by a code divisionmultiplexing mode. In this case, the code

${\mathbb{e}}^{j\; 2\;\pi\frac{u}{M}{\mathbb{i}}}$can be used, and the reference signals multiplied by the code aredelayed in the time domain.

In FIG. 16B, when a rank value is 4, reference signals are allocated bya code division multiplexing mode. Since a rank value is 4 and M factoris 1, reference signals of the respective ranks use all resources of onesymbol. In this case, since there is no available resource between thereference signals, the reference signals cannot be identified by thefrequency division multiplexing mode. Accordingly, the reference signalof each rank can be identified by the code only. In this case, thereference signals can be transmitted using SC-FDMA, OFDMA, etc.

Also, the control information can include information indicating a rankvalue for transmission. For example, when a physical antenna of an userequipment is 4, a rank value could be 1, 2, 3, or 4. In this case, for aspecific object, the base station needs to limit a rank value. Forexample, if multi-user MIMO (MU-MIMO) is supported, the base station canlimit a rank value applied to each user equipment so that userequipments grouped in one group use a single radio resource withoutcollision.

In this respect, since the uplink of the current 3GPP LTE system isbased on SC-FDMA of one antenna, the LTE-A system based on MIMO may needtransmission considering backward compatibility. Generally, it is knownthat MIMO is suitable for the OFDMA system. Accordingly, if the basestation limits a rank value for uplink transmission, SC-FDMA or OFDMAcan be selectively used depending on the limited rank value. Namely, incase of a user equipment that supports both SC-FDMA and OFDMA, if thelimited rank value is less than a predetermined value, the userequipment may use SC-FDMA scheme. If the limited rank value is more thanthe predetermined value, the user equipment may use OFDMA scheme. Forexample, for compatibility, even though the user equipment supports theOFDMA system when the rank value is greater than 1 (rank>1), the userequipment can support SC-FDMA when the rank value is basically equal to(rank=1). In this respect, it is considered that the user equipmentsupports SC-FDMA in case of rank=1 whereas the user equipment supportsOFDMA in case of rank>1. It is also considered that the user equipmentsupports SC-FDMA in case of rank=1 or 2 whereas the user equipmentsupports OFDMA in case of rank>2.

Furthermore, the control information can include information indicatingwhether the radio resource for the subframe has been allocated to asingle user or multiple users. Alternatively, the control informationcan include information indicating whether a MIMO mode is single-userMIMO (SU-MIMO) or multi-user MIMO (MU-MIMO). If the control informationindicates that the subframe has been allocated to a single user, thereference signal for each rank is only allocated and other resources canbe used for data transmission. Namely, in order to ensure orthogonalityto the reference signal of other user, it is not necessary to limit useof a specific resource or puncture the specific resource. On the otherhand, the control information can indicate that the radio resource forthe subframe has been allocated to multiple users. In this case, thereference signals of the respective ranks can be multiplexed within thesubframe considering the reference signals of other user equipmentallocated to which the same radio resource is allocated. The referencesignals of the respective ranks can be multiplexed by a frequencydivision multiplexing mode, a code division multiplexing mode, or theircombination. In order to multiplex the reference signals of therespective ranks with reference signals of other user equipments groupedin the same group, the control information can further includeinformation required to multiplex the reference signals of therespective ranks with the reference signals of other user equipments.For example, the control information can further include information fornot overlapping transmission locations or codes/sequences of thereference signals between the user equipments. For example, each userequipment may further need at least one of information related to a rankvalue of a user equipment which takes part in MU-MIMO, M factor,multiplexing mode, frequency offset, and code/sequence. In more detail,if the frequency division multiplexing mode is used, each user equipmentmay need information related to a rank value, M factor and frequencyoffset. If the code division multiplexing mode is used, each userequipment may need M factor and code/sequence.

FIG. 17 is a diagram illustrating an example of reference signalsallocated by a frequency division multiplexing (FDM) mode to implementMU MIMO in accordance with one embodiment of the present invention. InFIG. 17, U1 represents a reference signal of layer 1 of a first userequipment, and U2 represents a reference signal of layer 2 of the firstuser equipment. U1* represents a reference signal of layer 1 of a seconduser equipment, and U2* represents a reference signal of layer 2 of thesecond user equipment. Although the frequency division multiplexing modeis illustrated in FIG. 17, the reference signal pattern/mode forsupporting MU-MIMO may be defined even by the code division multiplexingmode as illustrated in FIG. 16.

Referring to (a) of FIG. 17, both the first user equipment and thesecond user equipment have a rank value of 1. The reference signals ofthe first and second user equipments have a frequency interval of 4.Accordingly, the M factors of the first and second user equipments arecommonly 4. However, the reference signals of the first and second userequipments are different from each other in their frequency offset.Based on subcarrier 0 (SC=0) in slot 1, the reference signals of thefirst and second user equipments have frequency offsets of 0 and 3,respectively (based on reference signal of layer 1). Accordingly, ifinformation related to a rank value, M factor and/or frequency offset isincluded in the control information related to reference allocation,multi-user MIMO can be implemented effectively. Moreover, each user mayneed further information on how to manage resources related withreference signals of other user, e.g. resources related with U1* and U2*in view of the first user equipment. In other words, each user may needfurther information on how to manage resources related with referencesignals of unallocated rank in terms of total transmission rank. Forexample, the first user equipment may need further information onwhether the resources related with U1* and U2* may be used to transmitits data or not. This further information may be transmitted using thecontrol information related to reference allocation or separately.

Referring to (b) of FIG. 17, the first user equipment and the seconduser equipment have rank values of 1 and 2, respectively. The referencesignals of the first and second user equipments have a frequencyinterval of 4. Accordingly, the M factors of the first and second userequipments are commonly 4. However, the reference signals of the firstand second user equipments are different from each other in theirfrequency offset. Based on subcarrier 0 (SC=0) in slot 1, the referencesignals of the first and second user equipments have frequency offsetsof 0 and 1, respectively (based on reference signal of layer 1).

Referring to (c) of FIG. 17, both the first user equipment and thesecond user equipment have a rank value of 2. The reference signals ofthe first and second user equipments have a frequency interval of 4.Accordingly, the M factors of the first and second user equipments arecommonly 4. However, the reference signals of the first and second userequipments are different from each other in their frequency offset.Based on subcarrier 0 (SC=0) in slot 1, the reference signals of thefirst and second user equipments have frequency offsets of 0 and 1,respectively (based on reference signal of layer 1).

FIG. 18A to FIG. 18D are diagrams illustrating examples of referencesignals allocated with regard to each rank by a frequency divisionmultiplexing (FDM) mode considering M factor in accordance with anotherembodiment of the present invention.

FIG. 18A to FIG. 18D are basically identical with FIG. 15A to FIG. 15Dexcept that reference signals are allocated to two OFDM symbols perslot. Accordingly, for description of FIG. 18A to FIG. 18D, refer to thedescription of FIG. 15A to FIG. 15D. However, referring to M=4 of FIG.18A, it is noted that when the M factor is 2, the reference signals arelocated in different subcarriers in l=1 and l=4 of slot 1 andcyclic-shifted in the frequency domain to obtain frequency diversity. Itis also noted that the reference signals are located in differentsubcarriers of slot 1 and slot 2 and cyclic-shifted in the frequencydomain to obtain frequency diversity. Namely, the reference signals canbe staggered in a frequency direction within a slot or subframe as seenfrom FIG. 18B to FIG. 18D.

If the reference signals are staggered within a subframe, they aretransmitted through four symbols so as not to overlap with one anotheron the frequency axis. In FIG. 18A, for understanding of the presentinvention, the reference signals are exemplarily cyclic-shifted in afrequency direction to obtain frequency diversity.

Accordingly, the locations of the symbols to which the reference signalsare shifted within a subframe or slot can be varied depending on acommunication environment under consideration. Also, the number ofreference signals transmitted through two symbols l=1, 4 of slot 1 orslot 2 can be varied. Referring to FIG. 18A with M=4, two referencesignals are transmitted through l=1, and one reference signal istransmitted through l=4. On the other hand, if one reference signal istransmitted through l=1, two reference signals can be transmittedthrough l=4. This is intended such that overhead does not exceed 15%.

FIG. 19 is a diagram illustrating examples of reference signalsallocated with regard to each rank by a code division multiplexing (CDM)mode considering M factor in accordance with another embodiment of thepresent invention.

FIG. 19 is basically identical with FIG. 16 except that referencesignals are allocated to two OFDM symbols per slot. Accordingly, fordescription of FIG. 19, refer to the description of FIG. 16. However,referring to FIG. 19, it is noted that when a rank value is 2, thereference signals are located in different subcarriers in l=1 and l=4 ofslot 1 and cyclic-shifted in the frequency domain to obtain frequencydiversity. It is also noted that the reference signals are located indifferent subcarriers of slot 1 and slot 2 and cyclic-shifted in thefrequency domain to obtain frequency diversity. Namely, the referencesignals can be staggered in a frequency direction within a slot orsubframe.

FIG. 20 is a diagram illustrating an example of reference signalsallocated by a frequency division multiplexing (FDM) mode to implementMU MIMO in accordance with another embodiment of the present invention.

FIG. 20 is basically identical with FIG. 17 except that referencesignals are allocated to two OFDM symbols per slot. Accordingly, fordescription of FIG. 20, refer to the description of FIG. 17. However,referring to FIG. 20, it is noted that the reference signals are locatedin different subcarriers in l=1 and l=4 of slot 1 and cyclic-shifted inthe frequency domain to obtain frequency diversity. It is also notedthat the reference signals are located in different subcarriers of slot1 and slot 2 and cyclic-shifted in the frequency domain to obtainfrequency diversity. Namely, the reference signals can be staggered in afrequency direction within a slot or subframe.

As described above, if the reference signals are transmitted using thefrequency division multiplexing mode, the reference signals of slot 2are cyclic-shifted in a frequency direction with regard to the referencesignals of slot 1, whereby frequency offset is given to the referencesignals between the two slots. The present invention suggests a hybridmethod of the frequency division multiplexing method and the codedivision multiplexing method. According to the hybrid method, since acode region is additionally provided, code offset using code index isadditionally suggested in addition to frequency offset using cyclicshift to the frequency axis. Namely, offset can additionally be given tothe reference signals allocated to slot 1 and slot 2, by usingcombination of cyclic shift to the frequency axis and code index.

FIG. 21 is a view showing a transmitter or receiver according to anembodiment of the present invention. The transmitter or the receiver maybe a portion of a BS or a UE.

Referring to FIG. 21, the transmitter or receiver 2100 includes aprocessor 2110, a memory 2120, an RF module 2130, a display module 2140,and a user interface module 2150. The transmitter or receiver 2100 isshown for convenience of description and some modules may be omitted oradded. In addition, the transmitter or receiver 2100 may further includenecessary modules. In addition, in the transmitter or receiver 2100,some modules may be subdivided. The processor 2120 is configured toperform the operation according to the embodiments of the presentinvention shown in the drawings. The detailed operation of the process2120 may refer to the contents described in FIGS. 1 to 20. The memory2120 is connected to the processor 2110 and stores an operating system,an application, a program code and data. The RF module 2130 is connectedto the processor 2110 and performs a function for converting a basebandsignal into a radio-frequency signal or converting a radio-frequencysignal into a baseband signal. For example, the RF module 2130 mayperform analog conversion, amplification, filtering and frequencyup-conversion or inverse processes thereof. The display module 2140 isconnected to the processor 2110 and displays a variety of information.The display unit 2140 includes, but is not limited to, known devicessuch as a Liquid Crystal Display (LCD), a Light Emitting Diode (LED) andan Organic Light Emitting Diode (OLED). The user interface module 2150is connected to the processor 2110 and may be configured by acombination of known user interfaces such as a keypad, a touch screenand the like.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. Moreover, it will be apparent that someclaims referring to specific claims may be combined with another claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, or theircombination. If the embodiment according to the present invention isimplemented by hardware, the random access method in the wirelesscommunication system according to the embodiment of the presentinvention may be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

If the embodiment according to the present invention is implemented byfirmware or software, the embodiment of the present invention can beimplemented by a type of a module, a procedure, or a function, whichperforms functions or operations described as above. A software code maybe stored in a memory unit and then may be driven by a processor. Thememory unit may be located inside or outside the processor to transmitand receive data to and from the processor through various means whichare well known.

The present invention can be applied to a method of performingcommunication in a wireless communication system. More specifically, thepresent invention can be applied to a method of transmitting referencesignals through multiple antennas in a wireless communication system.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

What is claimed is:
 1. A method of transmitting uplink (UL) signalsusing a plurality of subcarriers by a user equipment (UE) in a wirelesscommunication system, the method comprising: generating two or more ULdemodulation reference signals (DMRSs) for two or more data layersincluding a first layer and a second layer; mapping the two or more ULDMRSs to resource elements (REs) on an UL subframe per antenna portsused for transmitting the two or more data layers, transmitting the twoor more UL DMRSs, wherein when a rank is 3 or more, the two or more ULDMRSs are mapped non-continuously in a frequency domain, and aretransmitted based on an orthogonal frequency division multiple access(OFDMA) scheme, and wherein when the rank is 2 or less, the two or moreUL DMRSs are mapped continuously in the frequency domain, and aretransmitted based on a single carrier-frequency division multiple access(SC-FDMA) scheme.
 2. The method of claim 1, wherein the two or more ULDMRSs are staggered within the UL subframe or slot.
 3. The method ofclaim 1, wherein a first UL DMRS for the first layer and a second ULDMRS for the second layer among the two or more UL DMRSs are mapped to asame set of REs based on a combination of code division multiplexing andfrequency domain cyclic shifting, the two or more UL DMRSs mapped ononly one predetermined symbol per the each slot, and wherein the sameset of REs includes REs separated by N subcarriers per slot, and N isequal to 2, 3 or
 4. 4. The method of claim 1, wherein the two or more ULDMRSs mapped on a predetermined symbol cover all subcarriers of one ormore resource blocks.
 5. The method of claim 1, wherein the two or moreUL DMRSs are different from UL data and are precoded for ULmultiple-input and multiple-output (MIMO).
 6. A user equipment (UE) fortransmitting uplink signals using a plurality of subcarriers in awireless communication system, the UE comprising: a processor adapted togenerate two or more UL demodulation reference signals (DMRSs) for twoor more data layers including a first layer and a second layer, and tomap the two or more UL DMRSs to resource elements (REs) on an ULsubframe per antenna ports used for transmitting the two or more datalayers; and a transmitter adapted to transmit the two or more UL DMRSs,wherein when a rank is 3 or more, the two or more UL DMRSs are mappednon-continuously in a frequency domain, and are transmitted based on anorthogonal frequency division multiple access (OFDMA) scheme, andwherein when the rank is 2 or less, the two or more UL DMRSs are mappedcontinuously in the frequency domain, and are transmitted based on asingle carrier-frequency division multiple access (SC-FDMA) scheme. 7.The UE of claim 6, wherein the two or more UL DMRSs are staggered withinthe UL subframe or slot.
 8. The UE of claim 6, wherein a first UL DMRSfor the first layer and a second UL DMRS for the second layer among thetwo or more UL DMRSs are mapped to a same set of REs based on acombination of code division multiplexing and frequency domain cyclicshifting, the two or more UL DMRSs mapped on only one predeterminedsymbol per the each slot, and wherein the same set of REs includes REsseparated by N subcarriers per slot, and N is equal to 2, 3 or
 4. 9. TheUE of claim 6, wherein the two or more UL DMRSs mapped on apredetermined symbol cover all subcarriers of one or more resourceblocks.